i'm an old school stone user. the new machines cut all 3 angles at the same time.
9mm expandable pilot, 45° stone, grind until it cleans in the area you are going to use. 30° stone above the top edge of the 45, until it narrows the seat enough from the top, so that the outer edge of the valve face just overlaps (just a C hair) the top edge of the 45. narrow the 45 from the bottom with a 60° stone (if you don't have a hand cutter) until the seat is the same width all around, approx 1 to 1.5mm, exhaust slightly wider than intake, for better heat transfer. i prefer a hand cutter with a single blade for the bottom cut. it's easier to trim just where you want to, rather than let the stone take off metal where IT wants. stones tend to follow the hole. most of the time you have one side of the seat a lot narrower than the opposite side, before doing the bottom cut, and can't afford loosing any seat area on the narrow side.
that's simplified, on a good day. various problems can pop up to make the job more difficult.